1. Field of the Invention
The present invention relates to a contact material used for a commutator of a micromotor.
2. Description of the Prior Art
Conventionally, Au, Au alloys, Ag alloys or the like are used for a commutator of a micromotor, and in many cases, composites of these alloys, which are known as clad composite materials are used in order to improve a sliding characteristic during use of the motor. The clad composite materials include a material comprising a surface of a contact substrate having an Ag alloy layer cladded with Au or the Au alloy at its outermost layer for the purpose of mainly providing corrosion resistance.
Au or the Au alloy used fore outermost layer of the clad composite material is excellent in the corrosion resistance and contact resistance characteristics but has a disadvantage that high wear resistance cannot be ensured since the material is soft. For this reason, if such an Au or Au alloy clad composite material is used for the commutator of the micromotor, it is prone to wearing and tends to limit the life of the micromotor.
In view of the above, the present invention is directed to provide a contact material suitable for a commutator of a micromotor which can improve wear resistance and prolong the life of the micromotor by improving an Au-based clad composite material which has been conventionally used and is provided with Au or an Au alloy at its outermost layer.
In order to solve the above problem, the inventors studied various clad composite materials which are cladded with Au or Au alloys at their outermost layers, and have found an Au-based clad composite material described below which improves the wear resistance. That is the Au-based clad composite material comprising a surface of a contact substrate having an Agxe2x80x94Cuxe2x80x94Si alloy layer cladded with Au or the Au alloy, wherein an intermetallic compound Cuxe2x80x94Si is dispersed and deposited in Au or the Au alloy.
An Au-based clad composite material according to the present invention is a material such that a hard intermetallic compound Cuxe2x80x94Si is dispersed and deposited therein while an outermost layer, namely an Au or Au alloy layer maintains a sliding characteristics such as corrosion resistance or contact resistance characteristics which Au has at the same level as those of a conventional material, resulting in a substantial improvement in wear resistance. Consequently, the Au-based clad composite material according to the present invention provides a contact material suitable for a commutator of a micromotor.
The Au-based clad composite according to the present invention can be obtained by cladding a surface of a contact substrate having an Agxe2x80x94Cuxe2x80x94Si alloy layer with Au or the Au alloy and carrying out heat treatment at 500 to 800xc2x0 C. In the Agxe2x80x94Cuxe2x80x94Si alloy obtained by melting and casting, Cu is dissolved in Ag but Si is difficult to be dissolved in Ag, so that Si combines with Cu to form the intermetallic compound Cuxe2x80x94Si, which is dispersed and deposited in the Agxe2x80x94Cuxe2x80x94Si alloy. Cladding the surface of the Agxe2x80x94Cuxe2x80x94Si alloy in such a condition used as a substrate with Au or the Au alloy and carrying out the heat treatment for joining at 500 to 800xc2x0 C. cause the intermetallic compound Cuxe2x80x94Si in the Agxe2x80x94Cuxe2x80x94Si alloy to be dispersed in Au or the Au alloy. In the present invention, a wording is used that the intermetallic compound Cuxe2x80x94Si is dispersed and deposited in Au or the Au alloy, which represents the condition that Cuxe2x80x94Si in the Agxe2x80x94Cuxe2x80x94Si alloy of the substrate enters into Au or the Au alloy to be dispersed therein by the thermal diffusion.
When temperature of the heat treatment in this case is bellow 500xc2x0 C., the intermetallic compound Cuxe2x80x94Si is difficult to be dispersed and deposited in Au or the Au alloy and production efficiency is reduced since heat treatment time needs to be maintained long. When the temperature is above 800xc2x0 C., Ag reacts with Cu and the material itself begins melting.
Such a phenomenon of dispersion of the intermetallic compound Cuxe2x80x94Si in Au or the Au alloy by the thermal diffusion is confirmed for the first time as long as the inventors know. This phenomenon is found out in the combination of Auxe2x80x94Ag alloy with the Agxe2x80x94Cuxe2x80x94Si alloy. Having studied this phenomenon, the inventors presume that the Au-based clad composite material according to the present invention can be obtained when the following requirements are met. First, since the intermetallic compound Cuxe2x80x94Si is formed in the Agxe2x80x94Cuxe2x80x94Si alloy of the substrate, it is considered that the substrate alloy needs to include elements which easily forms an intermetallic compound with Cu. The element which forms the intermetallic compound with Cu is considered to be sufficient if it has nature of producing an Au alloy with a low melting point when combined with Au. Namely, for Si, it is considered that Si easily forms an intermetallic compound with Cu, and that when Au combines with Si, Si has the nature of producing Auxe2x80x94Si with a low melting point, thus providing the Au-based clad composite material according to the present invention.
The Au-based clad composite material according to the present invention is preferably a composite of Au or an alloy of Au and 1 to 70% by weight of Ag, and an alloy of Ag, 1 to 40% by weight of Cu and 0.05 to 10% by weight of Si. In the present invention, using Au as the outermost layer is better in terms of characteristics, but it is preferable to use the Auxe2x80x94Ag alloy when considering its cost. When using the Auxe2x80x94Ag alloy, a value of 70% by weight is recommended as an upper limit of Ag content since corrosion resistance which is a characteristic of Au is reduced when the Ag content exceeds the value. For the Agxe2x80x94Cuxe2x80x94Si alloy used as the substrate, the intermetallic compound Cuxe2x80x94Si becomes difficult to be formed when the Cu content is bellow 1%, and the contact resistance becomes higher when it is above 40% by weight, thus the above range of content is recommended. Further, for Si, an amount of the intermetallic compound Cuxe2x80x94Si to be formed is reduced when the Si content is bellow 0.05%, which makes it impossible for the intermetallic compound Cuxe2x80x94Si to be sufficiently dispersed and deposited in Au or the Au alloy. When Si exceeds 10% by weight, workability deteriorates and a process such as forming in a sheet shape or the like tends to be difficult which is required for forming the clad composite material.
In the Au-based clad composite material according to the present invention, the Agxe2x80x94Cuxe2x80x94Si alloy used as the substrate is cladded with Au or the Au alloy. The inventors, however, presume that the same effect can be obtained with an Agxe2x80x94Si alloy used as the substrate. Si in the substrate does not form the intermetallic compound with Ag, which differs from the above presumption by the inventors, but Si is dispersed and deposited in the Ag because it is difficult to be dissolved in Ag. Si in the substrate (Agxe2x80x94Si alloy) is considered to be dispersed and deposited in Au or the Au alloy which clads the same by thermal diffusion. Namely, forming the Au-based clad composite material by cladding the Agxe2x80x94Si alloy as the substrate with Au or the Au alloy causes Si to be dispersed or deposited in the outermost layer in a condition that sliding characteristics such as the corrosion resistance or contact resistance characteristics which Au has are maintained at the same level as those of the conventional material, resulting in substantial improvement of the wear resistance.
Thickness of Au or the Au alloy for cladding is preferably 0.1 to 20 xcexcm. Bellow 0.1 xcexcm, Au is dispersed in the substrate and the corrosion resistance of the outermost layer cannot be maintained. Above 20 xcexcm, the intermetallic compound Cuxe2x80x94Si is difficult to be sufficiently dispersed in Au or the Au alloy, which cannot achieve improvement of the wear resistance.
Using the Au-based clad composite material according to the present invention for the commutator of the micromotor can improve the wear resistance and thereby prolong the life of the motor. When using the Au alloy clad composite material according to the present invention for the commutator of the micromotor as described above, an Agxe2x80x94Pd alloy is preferably used for a brush which is a counterpart of the commutator. Forming the micromotor with such a combination can stably achieve prolonging the life of the micromotor.